Plant cell wall consists mainly of cellulose microfibrils embedded in a matrix of hemicellulose, pectin, and, in mature cells, lignin (Bacic et al. 1988). In addition, small amounts of enzymatic and structural proteins are present. In cereal grains, the hemicellulosic fraction of the cell wall consists mainly of arabinoxylan (AX). For example, a corn kernel consists of approximately 8% cell wall, ˜60% of which is AX. Arabinoxylan consists of a β-1,4-linked backbone of xylosyl residues that are substituted at varying intervals with arabinofuranosyl residues mainly through an α-(1→3) linkage. Glucuronosyl residues and acetyl groups further decorate AX at varying degrees, with the former attached mainly through and alpha-1,2-linkage and the latter with an O-2 or O-3 linkage (Dhugga 2007).
Arabinoxylan is known to be an antinutritional constituent of the corn grain feed targeted for monogastric animals, i.e., poultry and swine. Not only is it not digested, it is also known to reduce the rate of digestibility of other feed components, mainly because of its effect on the viscosity of the chime (Pettersson et al. 1990; Veldman and Vahl 1994). Aside from adversely affecting digestibility, arabinoxylan is the cause of sticky droppings and gummy excreta in monogastric animals. It is desirable, thus, to reduce the concentration of arabinoxylan in cereal cell wall, which will make it more suitable for feed applications.
Arabinoxylan is also a major component of corn stover, cereal straws, and switch grass (Carpita 1996). Because arabinose and xylose are five carbon sugars, currently used yeast fermentation procedure is not optimized to convert them into ethanol. Acetyl residues on AX are known to be inhibitory to the process of fermentation (Dhugga 2007). Corn accounts for nearly half of all the crop residue produced in the United States and will be a major source of cellulosic biomass when the process of ethanol production from the latter is streamlined (Dhugga 2007). A reduction in arabinoxylan and its substitution by cellulose in corn stover, cereal straws, and switch grass will be beneficial for ethanol production from these sources.
However, dietary fiber, particularly arabinoxylan, reduces cholesterol and low density lipoprotein levels in humans (WO 99/67,404). In breadmaking, bread quality depends heavily on the consistency of the dough. Dough that lacks viscosity alters the crumb structure of the bread and decreases the volume of bread produced. Arabinoxylan provides the viscous properties of dough (Girhammar et al. (1995) Food Hydrocolloids 9:133-140). Additionally, industries use isolated arabinoxylan preparations as thickeners, emulsifiers, or stabilizers in food, cosmetics, and pharmaceuticals. Therefore, in certain circumstances, it would be desirable to increase the concentration of arabinoxylans in plant.
Clearly, modulating the concentrations of polysaccharides, particularly arabinoxylan in various crops is a desirable goal. However, a direct approach using the enzymes that synthesize these polysaccharides has been hampered because of the difficulty in isolating and cloning the plant polysaccharide synthase genes. Polysaccharide synthase enzymes for the common polysaccharides are estimated to number in the hundreds. However, availability of the complete genome sequences of Arabidopsis and rice has made it possible to isolate the members of the cellulose synthase super gene family (Richmond and Somerville 2000; Hazen et al. 2002), which was accomplished through searching for homologous genes to a cellulose synthase gene that had previously been isolated from cotton (Pear et al. 1996). Proof for the involvement of any of these genes in β-glycan formation was obtained when a cellulose synthase-like (Csl) gene from guar that made β-mannan was functionally expressed in a heterologous system (Dhugga et al. 2004; Liepman et al. 2005).
An alternative approach was used by Burton et al. (2006) to identify mixed-linked glucan (MLG) synthase genes whereby they expressed the barley CslF sequences in Arabidopsis (Burton et al. 2006). MLG does not occur naturally in Arabidopsis walls. Detection of MLG by immunocytochemistry using antibodies specific to MLG in the walls of CslF-expressing Arabidopsis cells indicated the involvement of these sequences in MLG formation (Burton et al. 2006). Genes that encode xylan synthase, the enzyme that catalyzes the formation of the backbone of AX, remain unknown thus far. Compositions and methodologies useful in the modulation of arabinoxylan levels in plants are needed.